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Re: Using Select with arrays? (Relative newbie)
*To*: mathgroup at smc.vnet.net
*Subject*: [mg54234] Re: Using Select with arrays? (Relative newbie)
*From*: Curt Fischer <tentrillion at gmail.NOSPAM.com>
*Date*: Mon, 14 Feb 2005 00:57:50 -0500 (EST)
*References*: <cup5ip$dsr$1@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
Hugo Mallinson wrote:
> The subject might not be entirely correct, but here is my problem:
>
> I have a list of 5-variable data points like
>
> data = {
> {1, 1, string1, c, d}
> {1, 2, string2, c, d}
> {1, 3, string3, c, d}
> {1, 4, string4, c, d}
> {2, 1, string1, c, d}
> {2, 2, string2, c, d}
> {3, 1, string3, c, d}
> {4, 1, string2, c, d}
> {4, 2, string4, c, d}
> }
>
>
> and I want to extract just the points that have 1 (or 2, etc) as their
> first value. I think I should do something like
>
> Select[data, {1, _Integer, _String, _Integer, _Integer}]
>
> but that doesn't work.
None of your data matches the pattern you made. When you enter things
like string2 or c or d into Mathematica without quotation marks, they
are not strings or integers. You can easily find out what Mathematica
thinks they are.
In[1]:=
data = {{1, 1,
string1, c, d} , {1, 2, string2, c, d}, {1, 3,
string3, c, d}, {1, 4, string4, c, d} , {2, 1,
string1, c, d}, {2, 2, string2, c, d}, {3, 1,
string3, c, d} , {4, 1, string2, c, d}, {4, 2, string4, c, d}};
In[2]:=
Map[Head, data, {2}]
Out[2]=
{{Integer, Integer, Symbol, Symbol, Symbol}, {Integer, Integer, Symbol,
Symbol, Symbol}, {
Integer, Integer, Symbol, Symbol, Symbol}, {Integer, Integer, Symbol,
Symbol, Symbol}, {
Integer, Integer, Symbol, Symbol, Symbol}, {Integer, Integer, Symbol,
Symbol, Symbol}, {
Integer, Integer, Symbol, Symbol, Symbol}, {Integer, Integer, Symbol,
Symbol, Symbol}, {Integer, Integer, Symbol, Symbol, Symbol}}
That is, they are all Symbols. We can re-write your pattern to be more
flexible and we can use Cases[] instead of Select[]. (IME Cases[] is
easier to figure out.)
In[3]:=
Cases[data, {1, _Integer, _, _, _}]
Out[3]=
{{1, 1, string1, c, d}, {1, 2, string2, c, d}, {
1, 3, string3, c, d}, {1, 4, string4, c, d}}
So that gets you the preliminary result. But you want to extract the
member of this list that has the maximal second element.
> Having done that I need to find the maximum value of #2 for each
> string, which I presumably do by the same method as above to extract
> all string1 (or ...2) and then use Map[] and Max[]. I would do this all
> with For loops (revealing my lack of Mathematica chops :-) ) but I'd
> really like to learn how to do this sort of extraction in a
> Mathematica-y way. Any help would be greatly appreciated!
Let's say you want to know the maximum integer that occurs in slot #2
for all of the data that contain string3.
In[4]:=
Max[Part[#,2]&/@Cases[data,{_Integer,_Integer, string3,_,_}]]
Out[4]=
3
This code extracts the second part from all of the list elements that
match {_Integer, _Integer, string3, _,_} and finds the maximum value
thereof.
--
Curt Fischer
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